THE OXIDATION OF NI-NB ALLOYS AT LOW-OXYGEN PRESSURES AT 600-800-DEGREES-C

The oxidation of two Ni-Nb alloys containing 15 and 30 wt.% Nb has bee n studied at 600-800 degrees C in H-2-CO2 mixtures providing an oxygen pressure of 10(-24) arm at 600 degrees C and 10(-20) atm O-2 at 700 a nd 800 degrees C, these pressures being less than the dissociation pre ssure of nickel oxide. The scales formed on both alloys at 600 and 700 degrees C show only a region of internal oxidation composed of a mixt ure of alpha nickel and niobium oxides (Nb2O5 or/and NbO2), which form ed from both the metal phases present, ie., Ni8Nb and Ni3Nb. Only smal l, or even no, Nb depletion was observed in the alloys close to the in terface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800 degrees C produce d a continuous external scale of niobium oxides without internal oxida tion. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.